The strategy allows Obama to make sure his supporters – who may be discouraged by long lines after work on Election Day – actually cast their ballots, as well as to shore up support at a time when he’s leading in the polls in many states.

“It just makes sense as a campaign strategy because the public climate is very favorable to Obama right now,” says Darrell West, director of Governance Studies at the Brookings Institution. “The more people who decide now, the better off he is. You never know what’s going to change.”

So far, some 5 million people have cast early ballots in 13 states for which statistics are available. Thirty-four states now allow some form of early voting, and analysts expect that about one-third of all votes may be cast by Nov. 4. In some key states, like Colorado, where many people are casting their votes early by mail as well as in person, over 60 percent of ballots may be in before Election Day.

Although Republicans and Democrats are split fairly evenly in terms of ballots already received by the state, most of the mail-in ballots have yet to be turned in; registered Democrats have requested more mail-in ballots and Obama is currently leading in the polls.

“If these numbers start racking up, and McCain can’t change the trajectory of the campaign in Colorado, it’s just going to come to the point where he’s going to have to win 70 percent of the vote that’s remaining on Election Day, and that’s going to be impossible for him to do,” says Michael McDonald, a political scientist at George Mason University in Fairfax, Va. who studies early-voting trends.

Senator McCain has been urging his supporters to turn in early ballots as well, but not in so coordinated an effort and with seemingly less success. In Colorado, canvassers for Obama remind all voters they’re certain of to turn in their mail-in ballot right away, and the message seems to be sticking.

“We’ve had two or three people come to our door offering mail-in ballots,” says Luke Erikson, a recent PhD physics graduate who voted for Obama in Englewood, Colo., on Wednesday. He voted early since he didn’t want to make the lines any longer on Nov. 4, he says – though he still chose to vote in person because “I wanted the sticker.”

The most detailed data so far has come from North Carolina, a generally Republican state that has surprised many pundits by being in play this year, and where Democrats have outnumbered Republicans more than 2 to 1 in some 750,000 early ballots cast so far. In 2004, Democrats also had a slight edge in early voting, but the gap was much smaller – 49 percent to 37 percent.

Republicans still have an edge among traditional absentee voters, says Professor McDonald, but about 85 percent of the votes cast so far have been at special polling places, rather than by absentee ballot.

Meanwhile, in Florida – another key state for both campaigns – Democrats have outnumbered Republicans in in-person early votes by nearly 2 to 1, though Republicans reportedly have the edge in absentee ballots.

Some analysts caution against reading too much into the early numbers, noting that most votes are still cast on Election Day, and sometimes a campaign’s get-out-the-vote effort can be exhausted by then.

In Iowa in 2004, Democrats made a huge early-vote push, but then lost the state to Republicans when they skimped on Election Day activities, earning criticism for a faulty strategy. This time around, though, Obama’s extensive resources may make that less of a concern.

“I don’t think he has to worry about making a zero-sum decision about whether to do early voting or Election Day voting,” says Mr. McDonald. “I think he can do both.”

The trend has been moving toward early voting for some time, with more states adding it, in some form, as a way to reduce lines on Election Day and to ensure that those voters who have difficulty getting to polling locations during voting hours on a single day can still cast a ballot. Campaign strategists note that the traditional 72-hour get-out-the-vote push has started to become a grueling 720-hour push in many states.

Early voting has some downsides, including logistical challenges for states.

“One of the biggest drawbacks to early voting is that something happens in the week or a few days before the election that might lead you to want to change your vote,” says Lawrence Norden, director of the Voting Technology Project at the Brennan Center for Justice at the New York University Law School.

Still, Mr. Norden sees a lot of advantages, and he says that if Obama continues to be more effective at racking up early votes, “in two years or four years, I’d be very surprised if ... Republicans don’t make more of an effort to get more people to the polls early.”

In Littleton, Colo., on Wednesday, Christina Alexiades was pleased to be able to avoid crowds two weeks before Election Day. It was made even easier by an e-mail from the Obama campaign, reminding her to vote early and helping her find her nearest polling place.